feat: add ElevenLabs TTS provider

[nkmohit]

Summary

Adds ElevenLabs as a first-class TTS provider in the existing audio provider architecture. This gives users an additional high-quality English narration option, with provider-specific API handling, settings/UI integration, logo support, and a free-tier-safe fallback voice set for ElevenLabs accounts.

Related Issues

Closes #132

Changes

• Added elevenlabs-tts to the TTS provider type union
• Added ElevenLabs provider metadata to lib/audio/constants.ts
  • default base URL: https://api.elevenlabs.io/v1
  • supported formats: mp3, opus, pcm, wav, ulaw, alaw
  • provider-specific speed range: 0.7 to 1.2
• Implemented generateElevenLabsTTS(...) in lib/audio/tts-providers.ts
  • uses ElevenLabs xi-api-key auth
  • calls /v1/text-to-speech/{voice_id}
  • supports provider-specific output format mapping
• Added a curated free-tier-safe fallback voice set for ElevenLabs
  • 3 female, 3 male, 1 neutral
  • based on account-visible premade/default-style voices rather than library voices that fail on free tier
• Added ElevenLabs provider labels in settings i18n
• Added settings/store integration for ElevenLabs TTS provider config
• Added ElevenLabs logo asset in public/logos/elevenlabs.svg
• Added ElevenLabs request URL preview in TTS settings using the same simplified pattern as other providers

Type of Change

• New feature (non-breaking change that adds functionality)
• Bug fix (non-breaking change that fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/CD or build changes

Verification

Steps to reproduce / test

1. Open Settings → TTS
2. Select ElevenLabs TTS
3. Enter a valid ElevenLabs API key
4. Confirm the request URL preview shows:
   • https://api.elevenlabs.io/v1/text-to-speech
5. Run Test TTS
6. Confirm preview audio is generated successfully

What you personally verified

• Verified the provider is wired into the TTS registry, settings UI, i18n, and default settings store config
• Queried ElevenLabs voice inventory and switched to a safer free-tier fallback set based on account-visible premade/default-style voices
• Successfully got the ElevenLabs provider working locally after correcting provider configuration and voice selection

Evidence

• CI passes (pnpm check && pnpm lint && npx tsc --noEmit)
• Manually tested locally
• Screenshots / recordings attached (if UI changes)

Screenshots:

Logs of successful elevenlabs tts api call:

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have added/updated documentation as needed
• My changes do not introduce new warnings

[wyuc]

Reviewed, no issues found.